韧皮部通过胞间连丝负载的生物物理分析
Phloem loading through plasmodesmata: a biophysical analysis
摘要：
在许多物种中，蔗糖在离开叶片的途中从光合活性叶肉细胞经由胞间连丝沿其浓度梯度迁移到韧皮部，即，symplastically.在这些植物中的一些中，该过程完全是被动的，但在其他植物中，韧皮部蔗糖被主动转化为较大的糖，棉子糖和水苏糖，并被分离（捕获），从而将总韧皮部糖浓度提高到高于叶肉的水平。关于这两种共质体加载过程所赋予的机制和选择性优势，仍然存在问题。在这里，我们提出了一个综合模型-包括本地和全球运输和寡聚化的动力学-被动和主动共质体加载。我们还提出了通过胞间连丝运输的物理模型。利用这些模型，我们预测：1）相对于被动加载，即使在没有分离的情况下，韧皮部中蔗糖的低聚化也降低了达到给定输出速率所需的叶中糖含量，并且对于叶肉中给定浓度的蔗糖加速了输出;和2）对于生理上合理的参数值，可以实现低聚物的分离和总糖含量的反向梯度，但是在允许聚合物扩散回到叶肉中的情况下可以获得甚至更高的输出速率。我们讨论了这些预测，旨在澄清加载机制，健身的主动和被动共塑加载，和潜在的目标工程提高出口率的进一步研究。
英文摘要：
In many species, sucrose en route out of the leaf migrates from photosynthetically active mesophyll cells into the phloem down its concentration gradient via plasmodesmata, i.e., symplastically. In some of these plants the process is entirely passive, but in others phloem sucrose is actively converted into larger sugars, raffinose and stachyose, and segregated (trapped), thus raising total phloem sugar concentration to a level higher than in the mesophyll. Questions remain regarding the mechanisms and selective advantages conferred by both of these symplastic loading processes. Here we present an integrated model - including local and global transport and the kinetics of oligomerization - for passive and active symplastic loading. We also propose a physical model of transport through the plasmodesmata. With these models, we predict that: 1) relative to passive loading, oligomerization of sucrose in the phloem, even in the absence of segregation, lowers the sugar content in the leaf required to achieve a given export rate and accelerates export for a given concentration of sucrose in the mesophyll; and 2) segregation of oligomers and the inverted gradient of total sugar content can be achieved for physiologically reasonable parameter values, but even higher export rates can be accessed in scenarios in which polymers are allowed to diffuse back into the mesophyll. We discuss these predictions in relation to further studies aimed at the clarification of loading mechanisms, fitness of active and passive symplastic loading, and potential targets for engineering improved rates of export.
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